Ammonia versus Water Elimination in the Reaction of Diols with Urea under Metal Oxide Catalysis

This article features interesting and challenging reactions of urea, O═C(NH2)2, with biosourced diols, HO(CH2)nOH (n = 3, 4), driven by cheap and abundant heterogeneous catalysts. By tuning the catalyst and the reaction conditions, the hierarchical elimination of ammonia (that can be recovered) and water can be promoted between the −NH2 and HO-functionalities, giving rise to a variety of compounds under mild conditions. Thus, cyclic carbonates, linear mono- and diurethanes, cyclic urethanes, or even oligomeric urethanes, potential precursors of non-isocyanate polyurethanes (NIPURs), can be prepared, even with high yield and selectivity. A variety of catalysts have been tested. ZnO and CeO2 show high activity and quite different properties in promoting ammonia or water elimination. Their different behavior has been explained with the help of density functional theory (DFT) calculations. Contrary to ZnO that is dissolved as Zn(NH3)2(NCO)2 in the reaction medium, preventing the recovery of the oxide, CeO2 does not dissolve in the reaction mixture and can easily and quantitatively be recovered and reused. CeO2 is shown to be able to promote the sequential diol–urea coupling with the formation of oligo-urethanes.